1. Field of the Invention
The present invention relates to an electrically conductive casing for electronic devices formed using a fiber-reinforced plastic composition containing reinforcing carbon fibers having electrical conductivity and polyamides as matrix components.
2. Description of the Related Arts
Plastic molded articles have been widely used in casings for electronic devices, such as portable office automation (OA) devices, audio visual (AV) devices and portable phones. These casings require antistatic characteristics and electromagnetic shielding characteristics in order to prevent leakage of electromagnetic waves, which are generated in high-frequency circuits and, which affect other electronic parts and circuits, and to prevent invasion of electromagnetic waves from the outside which will cause erroneous operation of electronic circuits in the casings.
A typical method to furnish casings with antistatic characteristics is to coat antistatic paints and a typical electromagnetic wave shielding method is to plate the casing surfaces. These methods, however, cause an increase in the product costs due to an increase in the number of production steps. Further, the antistatic coating layer or the plating layer will be peeled off due to external forces, such as impact, when in use, resulting in a deterioration of the antistatic or shielding effects.
Additionally, portable electronic devices have spread rapidly in recent years. Although these portable devices must be lightweight, it is difficult to reduce the weight of conventional plastic casings because they have a thickness such that the minimum rigidity required for protecting the electronic devices is achieved.
In order to solve the above-mentioned problems, lightweight and rigid casings have been developed using composite materials, i.e., fiber-reinforced and especially carbon fiber-reinforced plastics. A typical example of materials used for such casings is disclosed in Japanese Patent Laid-open No. 4-167304 in which carbon particles and carbon fibers are compounded in a crystalline polymer. A warpage will occasionally occur, however, due to inhomogeneous crystallization as described in this patent, and it is difficult to use the composition for thin molded articles. A proposed method for solving this problem is to use polymer alloys in which amorphous resins are compounded into crystalline polymers. Since amorphous resins have generally low flowability, the polymer alloys also have low flowability compared with the sole use of crystalline polymers. It is therefore difficult to obtain thin-wall molded articles from polymer alloys due to increased molding pressures and temperatures. Widely used plastics include amorphous resins, such as polycarbonates and modified polyphenylene ethers, and semi-aromatic polyamides, which have a smaller mold shrinkage on molding and thus high dimensional accuracy, and a smaller warpage in thin molded articles. These amorphous resins, however, generally have-poor flowability, and both the amorphous resins and the semi-aromatic polyamides have low impact resistance when these resins are reinforced with carbon fibers.
The impact resistance will now be described in detail with reference to notebook-type personal computers which are provided with carbon fiber-reinforced plastic casings having high impact resistance. Notebook-type personal computers have weights in a range from 1 to 3 kg and require high electromagnetic wave shielding characteristics. Many electronic parts are packed into a casing to enable versatile usage, and thus these parts occupy a large volume. The casing therefore requires a reduction of weight and thickness. On the other hand, portable electronic devices must have high impact resistance, which is expressed as follows.
A box casing having a height of 20 to 50 mm, a length of 250 to 350 mm, a width of 160 to 260 mm, a thickness of 1.0 to 1.5 mm and a weight of W.sub.0 (g) is prepared in which boundaries between two side walls and between one side wall and the bottom wall have an inner curvature of 0.1 mmR or more and an outer curvature of 0.7 mmR or less and a thickness of 0.7 mm or more. A metal plate having a weight of W.sub.1 (g) and cut so as to come into contact with the four side walls of the casing is fixed to the bottom wall of the casing. When dropping the box casing onto a wood plate such that a corner between a boundary of two side walls and the bottom face is at the lowest position, the height forming casing cracks satisfies the following equation (I): EQU h&gt;45/(W.sub.0 +W.sub.1) (I)
For example, the height forming casing cracks is 4.5 cm or more for a notebook-type personal computer of 1 kg, or 2.25 cm or more for a computer of 2 kg. No electronic device casings made of conventional carbon fiber-reinforced plastic materials satisfysuchacriterion. Forexample,anotebook-typepersonal computer of 3 kg covered with a casing made of a carbon fiber-reinforced semi-aromatic polyamide has a height of 5 to 10 mm according to the above method.